1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling a power supply thereof. The image forming apparatus comprises a power storage unit that receives and stores electric power from an external power source such as a commercial power source, and a thermal fixing unit using electrothermally generated temperature. The image forming apparatus reduces usage of electric current from the commercial power source using electric power from both the commercial power source and the power storage unit when the thermal fixing unit is being warmed up. The image forming apparatus, in particular, has an energy saving standby mode, and the present invention relates to energy saving of power storage in the energy saving standby mode.
2. Description of the Related Art
An image forming apparatus is conventionally known, that forms a toner image on a recording medium using an electrophotographic process and comprises a fixing unit which feeds the medium with the image and presses it between temperature-controlled fixing parts, thus thermally fixing the image thereupon. A method has been proposed in the image forming apparatus, in which rising time of the fixing unit, and thus time until start of printing, is shortened by supplying an increased quantity of electric power to the fixing unit. For example, a power storage unit in the image forming apparatus is installed, and electric power from both a commercial power source and the power storage unit is used when powering up the fixing unit, according to the cited reference 1 (JPA 2002-174988).
Another proposal is to use excess electric power from the commercial power source to power up the fixing unit, by using electric power charged in the power storage unit during standby mode to drive a direct current motor when the image forming apparatus is making a copy, according to the cited reference 2 (JPU H07-41023).
Another method has been proposed, in which the power storage unit is combined with an inductively heated thermal fixing unit capable of adjusting the electric power inputted into the device from the commercial power source, and then the electric power inputted into the fixing unit is increased according to the degree of excess electric power from the commercial power source.
An on demand fixing unit including a heater such as the inductive heater or the ceramic heater, may achieve a faster start up of the image forming apparatus by supplementing the electric power supply because of a rapid rise in temperature in response to the electric power inputted thereto. Such an approach allows doing away with pre-warming the fixing unit while leaving the image forming apparatus in a ready mode, i.e., a mode in which the apparatus turns ready to receive printing within a predetermined time period. It is possible, in turn, to adopt an on demand fixing control method that avoids a loss of electric power resulting from maintaining a standby temperature therewith.
Aside from the on demand fixing control method in ready mode, power saving control such as the following examples is performed in the image forming apparatus in order to reduce standby power. The apparatus informs to a host that ready status is in the negative and then turns into the standby mode, when a set amount of time required before turning into an energy saving mode has been elapsed after stopping print operation. A backlight in a display panel and a power supply for running the display are switched off. A power supply to an optional unit is switched off. A power supply for control of such aspects as lighting of a photo interrupter is switched off. An actuator power supply is turned off for such purposes as switching off an idle state current of a DC motor. A power supply to a cooling fan is turned off. An operation of a switching power supply is suspended. Clock cycles of a controller logic circuit are decreased. Thus, controls in the energy saving standby mode are performed, which achieve energy saving controls in standby mode.
It is possible for a user to change the set amount of time required before turning into an energy saving mode, aside from an initial time value that has been determined by taking energy saving standards into consideration. It would also be possible that the amount of time required before turning into an energy saving mode is zero seconds, such that the image forming apparatus would enter energy saving standby mode immediately upon completing a print job.
Following is a description, with reference to the attached drawings, of a prior art of an image forming apparatus comprising a configuration that uses electric power from both the commercial power source and the power storage unit when powering up the fixing unit, and also having a energy saving standby mode.
FIG. 12 is a conceptual diagram that describes a hardware configuration of a power supply control as pertains to a conventional image forming apparatus.
Reference numeral 901 is a fixing unit, which is inductively heated when an AC power supply 904 supplies high-frequency electric power via a fixing power supply circuit 902, and is controlled to maintain a predetermined temperature by an image process controller 908. Reference numeral 903 is a two-converter low-voltage power supply, which receives electric power from the AC power supply 904 that is converted into primary DC voltage by a rectification circuit 905, and outputs a 5-volt DC power supply for control and a 24-volt DC power supply for an actuator via separate insulated-type DC/DC converters 906 and 907, respectively.
Reference numeral 908 is the image process controller, which communicates with an image forming controller 909, which is connected to a host 926, via a command signal 924. The image process controller 908 synchronizes a laser driver 910, which serves as an image forming material, a high-voltage power supply 914, and an actuator 915, which comprises such components as a scanner motor, a drum motor, a conveyor motor, and a solenoid fan motor, with a video signal 923, via a well-known electrophotographic process. The synchronization draws on information of a sensor type 912. A toner image is formed on a recording medium, and the image is pressed and fixed by the temperature controlled fixing unit 901. Reference numeral 927 is a backlight that illuminates a display panel 925.
Reference numeral 911 is a power saving switch, and reference numeral 922 is a signal to suspend the converter. If the image forming apparatus is not accessed via either the host 926 or the display panel 925 within a set period of time, the image forming controller 909 generates an energy saving command. Upon receipt of the energy saving command, the image process controller 908 performs power saving through interruptions and suspensions, and transitions to energy saving standby mode, with the ready signal in the negative.
Reference numeral 913 is a power storage unit, which is inserted in a supply route of the 24-volt output from the two-converter low-voltage power. In the power storage unit 913, a charging unit 917, for which the 24-volt power is provided, supplies a constant current to charge a capacitive unit 918, which is constituted of an electric double layer capacitor. The capacitive unit 918 is charged until a predetermined level of charged power is reached under control of a charged power monitoring unit 916. A power storage unit control signal 921, issued by the image process controller 908, stops the charging of the capacitive unit 918 and activates a DC/DC converter 919. A discharge switch 920 switches the power supplied to various loads for image forming from the 24-volt output from the two-converter low-voltage power to a 24-volt output boosted from an output of the capacitive unit 918 by the DC/DC converter 919. The electric power that is charged in the capacitive unit 918 is supplied as low-voltage power in place of the 24-volt output from the two-converter low-voltage power. Therefore, the AC power for the low-voltage power supply is allocated to the AC power for the power supply of the fixing unit.
The capacitive unit 918 must be charged to the necessary level of power in time for initiation of the next warm-up. Therefore, the charging rate is set to about 30 watts, which is on the order of 10% of the typical discharge rate, taking into account intermittent printing by a cold start.
FIG. 13 is a conceptual diagram that describes a hardware configuration of a charging component of the power storage unit 913.
Reference numeral 1001 is the charging component, and incorporates the charging unit 917, the capacitive unit 918, and the charged power monitoring unit 916, from FIG. 12. Reference numeral 1002 corresponds to the capacitive unit, which is configured of a capacitive unit with a fully charged voltage of five volts that makes two serial connections to the electric double layer capacitor. Conventionally, the capacitive unit is known as a secondary battery that uses an electrochemical reaction, from the standpoint of electric storage capacity, and as a capacitor, from the standpoint of number of charge and discharge cycles. According to the present application, however, it is necessary to satisfy both types of performance, and thus, the electric double layer capacitor is selected, with a capacity of several dozen farads, which is tremendously larger than even an electrolytic capacitor.
Whichever method is selected, the capacitive unit achieves power storage by charging as a charging characteristic until the predetermined voltage is reached, and thus, may configure a charge circuit of a given voltage with a power supply of a predetermined voltage, per Thevenin's theorem. A power supply per Thevenin's theorem is inefficient, however, because it is slow to charge. Thus, a typical charging unit of the capacitive unit is configured of a charge circuit of a given current, employing a power supply of a higher voltage than the fully charged voltage in order to charge the capacitive unit within the time predetermined for the purpose, as well as a chopper control in order to achieve highly efficient conversion.
According to this prior art, the 24-volt power supply is routed via a switching unit 1004 and a choke coil 1003 to the capacitive unit 1002. The voltage of the charging current is converted in a current detection resistor 1007, compares the charging current voltage with a baseline power supply 1009 in a comparator 1008, and pulses at a gate of a one-shot multi-vibrator 1006. The one-shot pulse switches off the switching unit 1004, and transmits the current, with which the choke coil 1003 is charged by a circulating diode 1005 connected to a ground, to the capacitive unit 1002. The chopping circuit of a given current that is switched off for a given time is thus configured into the charging unit.
A hysteresis converter 1010 uses a baseline power supply 1011 to detect the voltage of the electric double layer capacitor, in order to avoid excess charging. The charged power monitoring unit is configured such that, when the detected voltage reaches five volts, a set terminal of the one-shot multi-vibrator 1006 is switched on, forcibly stopping the charge. Reference numeral 1012 is an OR gate, which inputs a forcible charge stop signal from the image process controller 908 into the set terminal of the one-shot multi-vibrator 1006.
FIG. 14 is a diagram describing power storage control at the time of printing, as pertains to the prior arts depicted in the configurations in FIGS. 12 and 13.
Reference numeral 1101 is a fixing temperature, reference numeral 1102 is the incoming AC power, reference numeral 1103 is power accumulated in the capacitor, and reference numeral 1104 is power coming into, and discharging from, the capacitor. The indicators display the charge and discharge operation of the capacitive unit for the intermittent printing that can occur during a cold start, along the same temporal axis. Reference numeral 1107 is a first discharge section and reference numeral 1108 is a second discharge section. The capacitor switches to discharge in section when activating the drum motor drives power consumption to a peak, once either of a predetermined time 1105 or 1106 has passed. Approximately 300 watts of electric power is supplied as 24-volt power to a 24-volt load. When the discharge is completed, power accumulated in the capacitor drops below a level for commencing charging 1112, thus causing charging to take place in either a charging section 1109 or 1110 until a maximum capacitance level 1111 is reached.
The level for commencing charging 1112 is set to a first capacitance level that corresponds to a power level that is required of the capacitive unit for warm-up, and the maximum capacitance level 1111 is set to a second capacitance level that corresponds to a maximum capacitance level of the capacitive unit. A monitoring capacitance level of the charged power monitoring unit maintains the capacitance level between the first capacitance level and the second capacitance level.
The charging rate of the charging unit of the power storage unit is set to a first charging rate, which is capable of reaching the first charged power level and is determined based on time period from an end of a first warm-up, of a first intermittent print at cold start, to a start of a second warm-up, together with the amount of the discharge. The charging rate is set to about 30 watts, which is on the order of 10% of the typical discharge rate.
FIG. 15 describes power storage control as pertains to energy saving standby mode in the configuration in FIGS. 12 and 13. Items in FIG. 15 that describe events identical to events in FIG. 14 are designated with identical reference numerals, and descriptions thereof are omitted.
Reference numeral 1201 is an expanded representation of the charging of the power coming into, and discharging from, the power storage unit. Reference numeral 1203 is section of the energy saving standby mode, in which the energy saving unit suspends power to the actuator and so on at an energy saving command generation timing 1202. The configuration is intended to reduce power consumption only with the idle state power of the 5-volt DC power supply for control insulated-type DC/DC converter 906 of the two-converter low-voltage power supply 903, and the sleep power and the capacitive unit's charging power of the image process controller 908 and the image forming controller 909.
Even though power is being conserved in energy saving standby mode by powering off at predetermined loads according to the above prior arts, a charging power consumption 1207 of approximately 30 watts occurs during charging of the capacitive unit in a first charge section 1204 of the energy saving standby mode. A power consumption similar to the charging power consumption 1207 occurs even in a second charge section 1206 of the energy saving standby mode, which is a recharge operation resulting from a natural discharge of the capacitive unit. The charging power consumption in the first or second charge section is in the range of 35 watts, as compared with a normal power consumption in the range of 5 watts in a non-charge section 1205 of the energy saving standby mode, and thereby a spike arises in power consumption.
While this power consumption is small as average power consumption because of the short duration, the amount of power involved may exceed energy saving standards. It is therefore necessary as a countermeasure to stop charging during the energy saving standby mode.
Stopping recharging during the energy saving standby mode, however, causes both a natural discharge of the capacitive unit and a cool down of the fixing unit being left unattended. Consequently, either the capacitive unit is recharged or the fixing unit is warmed up without any assistance from the capacitive unit, when recovering from the energy saving standby mode to the ready mode. In either case, more time is required to recover the image forming apparatus to a print-ready mode, and thereby resulting in a loss of the on demand capability thereof.
In particular, when setting a time period short before switching to the energy saving mode, the frequency increases with which the image forming apparatus switches to the energy saving standby mode without the charged level of the capacitive unit being fully recharged. Hence, the method in which the charging of the capacitive unit is stopped in the energy saving standby mode fails to maintain the on demand capability.